Lining for acid tanks and method of use



Sept. 13, 1966 T. E. SAXMAN LINING FOR ACID TANKS AND METHOD OF USE Filed May 5, 1965 INVENTOR. I THEODORE E. SA XMAN Q QMZJJW United States Patent Goodrich Company, New York, N.Y., a corporation of New York Filed May 5, 1965, Ser. No. 453,446 7 Claims. (Cl. 206-2) This invention relates to construction and use of tanks for strong acids or other highly corrosive liquids such as are used in the pickling of metals to remove surface oxides from the metals before further processing.

The invention can be used for both batchwise pickling of metal objects and for continuous pickling of metal in sheet or strip form, as well as for other operations requiring the handling of objectsin highly corrosive liquids, such as electroplating.

Dilute sulfuric or muriatic acid such as is used in pickling of ordinary iron and steel can be satisfactorily contained in metal tanks provided with heavy rubber linings, particularly when provided with an internal brick sheathing, as shown in Fritz Patent 1,899,413. Stronger acids are required for other metals and alloys such as stainless steel, copper or brass, or even aluminum, and it is common to use either nitric acid or a mixture of 16% nitric and 4% hydrofluoric acid for pickling metals. Sometimes the same tank will be used sometimes with one and sometimes with the other of these acid solutions.

Nitric acid will attack and will more or less rapidly destroy most organic materials, including rubber. Consequently, it has been the practice to avoid rubber linings for nitric acid service and use instead a chemically saturated lining material such as polyvinyl chloride which will resist the oxidizing action of nitric acid for prolonged periods of time, as reported in Industrial and Engineering Chemistry, vol. 31, p. 968, August 1939. Such linings are commonly given mechanical protection by an internal sheathing of carbon brick.

Experience has shown that even though the vinyl lining material itself is not disintegrated by the acid, the linings tend to have a relatively short life for one reason or another, such as penetration through the seams resulting from incomplete fusion of the joints in the sheets of vinyl material from which the linings are fabricated. Even the smallest flaw in a seam may permit the powerful acid to penetrate the lining sooner or later and then to attack the steel shell and perforate the tank.

The object of this invention accordingly is to provide a tank for containing the most powerful acids used for metal treatment for much longer periods of time than has heretofore been possible before replacement is required.

This object is accomplished by providing a steel tank shell with a lining of hard rubber, preferably covering the hard rubber with a suitable plastic film material, and finally protecting the lining with an internal sheathing of bricks, preferably carbon bricks. This construction is then preferably pretreated with a liquid which is essentially inert to the lining materials before the corrosive acid is introduced.

This invention will be described with reference to the accompanying drawing showing a schematic section through a lined tank.

In the embodiment illustrated in the drawing, a seamless steel tank shell 1 is fabricated in the usual way, by welding of steel plates. After the usual cleaning and cementing, the shell is lined with sheets of unvulcanized hard rubber 2, with joints as smooth as is practicable to avoid interference with the subsequently applied brick sheathing. The lining is then vulcanized in the usual way to convert the rubber to the hard rubber condition.

The exposed surface of the hard rubber is next preferably covered with sheets of plastic film material 3 held in place by a suitable adhesive cement. The plastic film material may be polyvinyl chloride or a copolymer consisting primarilyof vinyl chloride and may contain a small proportion of plastizicer such as dioctyl phthalate or a polyester of higher molecular Weight or even a chlorinated or nitrated hydrocarbon plasticizer. It is not essential that the film be completely resistant-to the acid, for reasons which will be explained below, and other types of film material such as vinylidene chloride, polyethylene, chlorinated polyethylene, or even othersmay be substituted if desired. It is preferred to use two layers of film material with staggered joints to avoid penetration of the acid at the seams, although a single layerwith lapped joints may be used, since the film material will be quite thin so that the extra thickness of the overlaps at the joints will not be noticeable and will not present any problem in the application of the brick sheathing. The filmmaterial is preferably held in place by a contact adhesive, such as a neoprene cement; although other reasonably acid-resistantadhesives can be used instead.

The tank construction is completed'by application ofan internal sheathing made preferably of carbon bricks 4 bonded with a sulfur cement or other mortar not affected by nitric and hydrofluoric acids.

To obtain best service life from the tank, the brick sheathing should be soaked in an inert liquid before the tank is placed in service. This may be accomplished simply by filling the tank With water and letting it stand for suflicient hours or days to permit the water to penetrate completely through the pores of the sheathing bricks. Even better results will be obtained if the liquid with which the brick sheathing is soaked is a material which will neutralize or insome other way prevent or retard the penetration of the acid through the pores of the brick to the surface of the lining and the chemical attack onthe lining and diffusion of acid through the lining to the metal shell. Such materials may be an alkali solution which will convert such' acid as may diffuse into-the pores into a salt, or it may be an oxidizable material which will convert the nitric acid to the less corrosive nitrous acid, or a compound which will react with the acid to precipitate a solid for filling the pores such as a water glass solution. Nevertheless, it will often be sulficient to use simply water which will be most convenient as it will not need to be entirely removed for storage or discard, but only enough will need to be removed after the preliminary soaking period to permit introduction of the required quantity of strong acid to bring the contents of the tank to the required strength for use.

Acid tanks of the kind described herein can be used to contain even the strongest and most destructive acids for unusually long period of years, particularly if presoaked in an inert liquid as described above, for the following reasons:

The ultimate failure of a lined tank, which generally means perforation so that the contents begin to leak, is the result either of destruction of the lining or of diffusion of the corrosive liquid through the lining so that it will attack the tank shell. Consequently, it is not enough to guard against only one of these occurrences but both must be prevented.

Mechanical destruction of the lining is normally prevented by the presence of the internal brick sheathing which prevents metal objects handled in the tank from gouging the lining. In addition, the hard rubber lining becomes essentially a single unit as the result of the adhesiveness of the rubber in its unvulcanized condition when it is applied and of the chemical reaction which occurs during vulcanization, so that the seams are far less likely to be affected by relative motion with respect to the brick sheathing resulting from differential thermal expansion than the seams in a plastic lining material.

Chemical destruction of the lining in this particular construction is prevented by the specific combination of materials together with the presoaking of the brick.

The hard rubber which constitutes the major part of the thickness of the lining is a chemically saturated material which is inherently resistant to chemical attack. It is further protected by the surface layer of plastic film material, especially if the latter is a material which is almost completely inert to strong oxidizing acids, such as the preferred polyvinyl chloride film material.

Moreover, effective contact of the acid with the lining is either entirely prevented or greatly retarded by the presoaking of the brick sheathing. The presoaking prevents any contact of the acid with the lining material on initial filling of the tank, since the pores of the brick sheathing will be filled with the inert liquid and the acid cannot reach the lining except by a process of slow dilfusion. Without such presoaking, the heavy strong acid sinking to the bottom of the tank would go directly through the empty pores of the bricks and start an immediate attack on the lining material. Consequently, the presoaking not only greatly retards any penetration of the acid to the lining, but entirely prevents penetration of the strong undiluted acid such as would otherwise occur when the tank is first filled and before the contents are mixed.

In addition, when the acid finally does slowly diffuse through the thickness of the brick sheathing, it is greatly diluted by the inert liquid with Which the pores had been soaked. This extremely dilute acid is capable of attacking the lining only very slowly, with the result that such decomposition products as may be formed will tend to fill the pores and prevent further diffusion and further chemical attack of the lining.

Finally, diffusion of acids through the thickness of the hard rubber lining is found to be very slow indeed, so that the lining can be expected to remain serviceable for a great many years.

The combination of all of these factors gives assurance against penetration of the corrosive acids resulting either from destruction of the lining or from diffusion for such extremely long periods that the useful life of the lined tank will be extended for many more years than has heretofore been considered to be possible, even in this most severe type of service involving nitric acid or mixtures of nitric and hydrofluoric acid.

I claim:

1. An acid tank comprising a metal shell, a lining of hard rubber bonded to the shell, plastic film material adhered to the hard rubber lining by an acid-resistant adhesive, and an internal sheathing of acid-proof brick.

2. A tank as defined in claim 1, in which the internal sheathing is of carbon brick.

3. An acid tank comprising a metal shell, a lining of hard rubber bonded to the shell, polyvinyl chloride film material adhered to the hard rubber lining by a neoprene adhesive, and an internal sheathing of carbon brick.

4. The method of preparing a tank for nitric acid service which comprises lining a metal tank shell with hard rubber, applying an internal brick sheathing bonded with acid-resistant mortar, and soaking the lined tank with an inert aqueous liquid to fill the pores of the brick with the inert liquid before introduction of the nitric acid.

5. The method defined in claim 4, in which the hard rubber lining is covered with a plastic film material before application of the brick sheathing.

6. The method defined in claim 4, in which the inert liquid is water.

7. The method of preparing a tank for nitric acid service which comprises lining a metal tank with hard rubber, covering the hard rubber lining with a plastic film material adhered by an acid-resistant adhesive, applying an internal sheathing of carbon brick bonded with acidresistant mortar, and soaking the lined tank with water to fill the pores of the brick with water before introduction of the nitric acid.

References Cited by the Examiner UNITED STATES PATENTS 1,202,457 10/1916 Wales. 1,899,413 2/1933 Fritz 206-2 2,054,587 9/1936 Neuhaus.

LOUIS G. MANCENE, Primary Examiner. 

3. AN ACID TANK COMPRISING A METAL SHELL, A LINING OF HARD RUBBER BONDED TO THE SHELL, POLYVINYL CHLORIDE FILM MATERIAL ADHERED TO THE HARD RUBBER LINING BY A NEOPRENE ADHESIVE, AND AN INTERNAL SHEATHING OF CARBON BRICK. 